Water quality for regeneration

Water quality for regeneration

• The tables show the minimum water quality required for each regeneration step in a softener and in a water demineralisation plant.
• CFR: In general, water treated by the previous column is used for dilution and slow rinse (displacement).
• RFR: In general, water treated by the column itself is used for dilution and slow rinse (displacement).
• MB (polishing or CPP): use demin water from the upstream demin unit, or clean condensate.
• MB ("working"): use demin water from the treated water storage tank or from a twin unit.
• For the backwash of Amberpack systems, treated water is recommended.
• In practice, decationised water is often replaced by demineralised water (see below).

1. Softening: Quality of water required for each regeneration step

Step	Reverse flow regeneration	Co-flow regeneration
Backwashing	Feed water	Feed water
Compaction	Soft water
Dilution	Soft water	Feed water
Displacement	Soft water	Feed water
Final rinse	Feed water	Feed water

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2. Demineralisation: Quality of water required for each regeneration step

Step	Reverse flow regeneration		Co-flow regeneration
	Cation	Anion	Cation	Anion
Backwashing	Feed water	Decationised	Feed w.	Decationised
Compaction	Demin. or decat.	Demineralised
Dilution	Demin. or decat.	Demineralised	Feed w.	Decat. or demin.
Displacement	Demin. or decat.	Demineralised	Feed w.	Decat. or demin.
Final rinse	Feed water	Decationised	Feed w.	Decationised

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The reasons are:

• Backwash: backwashing anion resins in the OH form with feed water may cause calcium carbonate precipitation (see below). For cation resins in the H form, raw water brings some risk, as bicarbonate may convert co carbon dioxide, create bubbles and force some resin to float. Resins regenerated in reverse flow are not backwashed every cycle.
• Compaction: this step is only required in Upcore (Amberpack Reverse) and Stratapack systems and is considered in IXCalc as a very short backwash; it requires the type of water used for regenerant dilution.
• NaOH dilution (CFR): if the raw water contains any hardness, you can't use raw water to dilute NaOH otherwise precipitation may occur (bicarbonate in raw water is converted to carbonate and CaCO3 has a very low solubility).
• Acid dilution (RFR): if the dilution water contains any cations, you spoil the highly regenerated part of the resin and the treated water quality drops.
• Brine dilution (RFR): for softening units, the same reasoning applies as for acid dilution of demineralisers.
• NaOH Dilution (RFR): the same reasoning applies as for acid dilution. Therefore demineralised water must be used.
• Displacement rinse (CFR & RFR): the water used there is always the same water as that used for regenerant dilution. In general, the flow rate used for dilution water during regenerant injection is maintained, and the dosing pump of the concentrated regenerant is merely stopped.
• Demin. or decat.? Using demineralised water (from the demin. water storage tank) for the dilution and displacement of acid in reverse flow regeneration is normal. Taking decationised water from another line is usually not considered a practical option. The same applies to the dilution and displacement of caustic soda in co-flow regeneration, and allows for simultaneous regeneration of the cation and anion resins. Otherwise cation must be regenerated first so that decationised water is available for regeneration of the anion. Simultaneous regeneration makes the process almost 50 % shorter. 
  In IXCalc, the following assumptions are made:
  • RFR: the acid dilution and displacement water is demineralised
  • CFR: the caustic dilution and displacement water is decationised
  For softening, nitrate removal and other applications using NaCl, treated water should be used for regenerant dilution and displacement when a low leakage is required and regeneration is done in reverse flow. When the leakage specification is not critical, feed water can be used. However, when you use feed water, you contaminate the resin layers that should be totally regenerated to obtain the lowest leakage.
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